Thermoplastic resin compositions used in fields such as automobile materials, home electric appliances and general merchandise are demanded to have a good balance between mechanical strength such as impact resistance and rigidity, and moldability. Therefore, rubbery polymer-containing rubber-reinforced thermoplastic resin compositions represented by high-impact polystyrenes (HIPS) and acrylonitrile-butadiene-styrene (ABS) resins are widely utilized.
Meanwhile, from the viewpoint of achieving carbon neutrality, there is an increasing need for a resin composition utilizing a naturally-occurring material represented by polylactic acid. One example of application of a naturally-occurring material to a rubbery polymer-containing thermoplastic resin composition is a technology that substitutes a synthetic rubber used as a rubbery polymer such as polybutadiene, with a natural rubber.
WO 2011/132795 describes a rubbery polymer-containing ABS resin comprising a deproteinized natural rubber having a nitrogen content of 0.1% by mass or less and a synthetic rubber. However, WO '795 explains that deproteinization of natural rubber latex by a known deproteinization method is necessary because impurities contained in natural rubber, particularly proteins, interfere with graft polymerization reaction. Furthermore, WO '795 explains that a high impact resistance-improving effect could not be obtained by using undeproteinized natural rubber latex. As natural rubber deproteinization methods, specifically, a method of degrading proteins by an addition of a proteinase to a natural rubber latex (JP H6-56902 A), a method of washing a natural rubber with a surfactant (JP 2004-99696 A) and a method of degrading proteins by an addition of an urea-based protein denaturant to a natural rubber latex (JP 2009-84333 A) have been disclosed.
Deproteinization of a natural rubber is a complicated process. Therefore, practical application of a deproteinized natural rubber as a rubbery polymer (R) of a rubber-reinforced thermoplastic resin composition such as HIPS or ABS resin is difficult at an industrial level. In view of this, it could be helpful to provide a natural rubber-containing thermoplastic resin composition having an excellent balance of physical properties in terms of impact resistance, fluidity and color tone, by using a natural rubber latex, but without requiring a complicated deproteinization treatment of a natural rubber.